The degree of plasticity in axons innervating various laminae of the rabbit superior colliculus will be investigated. Neonatal lesions will be placed in one of the visual, auditory or somatosensory tracts supplying the superior colliculus, and after 6 months a second lesion will be placed in another tract. Degeneration resultant from the second lesion will be revealed by the Nauta and Fink-Heimer techniques, and any abnormal distribution of axons carefully plotted. In this manner, it will be possible to determine if axons respond to the removal of a neighboring group of axons by spreading into the vacated space. The effects of physical distance, cross-modality, and cortical vs. subcortical axons will be assessed by the experiments proposed here. This study will attempt to determine systematically the potential for axonal plasticity in response to nearby deafferentation in a mammal, and try to determine some of the factors which limit this phenomenon.